1. Field of the Invention
The present invention relates generally to tunable electro-optical reflectarrays and, more particularly, to tunable infra-red reflectarrays and to several particular tunable infra-red reflectarray structures.
2. Description of Related Art
Reflectarrays are traditionally passive, planar microstrip antenna arrays designed for discrete reflected phase manipulation at each individual antenna element making up the array. By spatially varying the phase response of the antenna array, reflectarrays allow a planar surface to impress a non-planar phasefront on the reflected radiation, for example, a spherical wavefront. Initially proposed as a low-cost replacement for mechanically bulky parabolic reflectors, reflectarrays have been successfully developed and utilized at both radio (RF) and millimeter-wave (mmW) frequencies.
From basic reflectarray theory, the phase response of a single reflectarray element may be regulated by three core factors: the response of the ground plane, the response of the patch element by itself, and inter-element coupling between the element and the array. Patch element size and coupling parameters are difficult to modify post-fabrication, although techniques are available to modify the response of the groundplane. Furthermore, it is preferable to perform any type of modification beneath the groundplane to maintain as high of a system reflectivity as possible.
By definition, infrared reflectarrays consist of an array of resonant, sub-wavelength microstrip elements which, when excited, re-radiate in a specified pattern with an arbitrary phase front. Thus, it is possible for a physically flat reflectarray to provide focusing or collimation with a potentially smaller footprint and weight than an analogous polished element. In terms of behavior, reflectarrays share many similarities to phased arrays with each microstrip element of the array resonating with a progressive phase difference to achieve a desired far-field directivity.
An embodiment of a conventional static infra-red reflectarray is depicted in FIG. 1. The active elements of the reflectarray 101, 111, 121 each have an associated phase response 111, 115, 125 based on the particular composition, electrical properties, and spatial orientation of the element. The elements are separated from a ground plane 130 by a stand-off layer 137.
For infrared reflectarrays currently in development, elements and ground planes may be comprised of gold or aluminum and the stand-off layer may be comprised of Zirconium Dioxide or a commercially available spin on polymer such as benzocyclobutene (BCB). The resonant phase delay of each discrete element upon illumination may be determined by a number of factors:                (1) Material properties of the elements, ground plane, or standoff layer;        (2) Shape of the elements (currently patches are used, however other shapes may be employed for specific effects or ranges of effect);        (3) spacing/periodicity of the elements;        (4) height of the stand-off layer; and        (5) element dimensions.        
Similar to RF designs, most static infrared reflectarrays only vary element dimensions, both for ease of design and ease of fabrication. When a static reflectarray is radiated with a plane wave of uniform phase front, all of the elements will be excited in phase, but will re-radiate out of phase, with a designed delay given by the dimensions of the element.
In the case of patch elements, re-radiation by individual elements may be directed, fairly uniformly, in all directions away from the ground plane. Due to differences in phase response of the elements in the array; however, re-radiation in the far-field may only occur in a specified direction (neglecting side or grating lobes) due to interference between all of the elements. In configurations where the main lobe is directed off of the normal, specific lobe tilt may be determined by the distance between the elements and the progressive phase difference between the elements.
A reflectarray's performance may be improved by reducing the reflection loss of patch elements. This may be accomplished by increasing the thickness of the substrate layer to reduce loss introduced by the band gap of the resonant patches while increasing the bandwidth of the reflectarray element. This increase in substrate thickness may, however, cause a decrease in overall phase variation. This may be mitigated by using patches loaded with particular arrangements of slots.
Reflectarray layouts, in the case of normally incident collimated or spherical wave illumination, follow a layout analogous to a graded Fresnel zone plate (FZP). For the reflectarray, elements may be placed into periodic zones expanded out from the center of the device based on the element's phase. In a particular embodiment of a reflectarray, it was calculated that a phase step size of 45 degrees per element would be required to achieve 95% focusing efficiency into the primary zone. More complicated embodiments; such as for designs requiring off-axis illumination, would require more intricate layouts.
To provide a progressive phasing across the surface of the reflectarray to impose a spherical phase front, a layout generator may be required to place reflectarray elements for optimal performance. Such a generator may be accomplished via a software program or a hardware device.
One readily apparent disadvantage of existing reflectarray technology is that the reflectarray, once fabricated, generates one particular reflection pattern. A reflectarray whose particular individual elements may be dynamically tuned would allow for variation in reflected phase fronts and therefore provide capabilities such as beam steering. Although available for radio-frequency (RF) wavelengths, such reflectarrays have so far been realized by loading each individual element into the array to effectively vary their dimensions dynamically. Specifically, in the radio-frequency band, reflectarray elements may be individually loaded with discrete electronics. This approach is not feasible for reflectarrays with large numbers of elements, such as infrared reflectarrays, given that an infrared reflectarray may have on the order of 10 million elements with element sizes less than 5 microns, which all must be loaded and tuned.
The realization of an effective tunable infrared reflectarray presents special challenges as most of the methods used to create tunable millimeter wave reflectarray fail in the IR frequency range due to RC time constant limitations, antenna size restrictions, optical/electrical properties of materials at IR frequencies, or other problems related to the frequency.